The invention relates to a hydrostatic transmission having two bent axis-type axial piston units which are operated alternately as a pump and as a motor, with the two axial piston units being mounted in a common pivotable dual yoke and being pivotable together with the latter, as per the preamble of Claim 1.
The invention also relates to a hydrostatic transmission having two bent axis-type axial piston units which are operated alternately as a pump and as a motor, with the two axial piston units having a common pivotable dual swashplate with contact faces which are inclined with respect to the shaft of the respective axial piston unit to different extents, as per the preamble of Claim 4.
Hydrostatic transmissions are the subject matter of the prior application DE 10 2006 025 347. They are used in particular in power-split transmissions.
Bent axis units have in each case a cylinder drum which is mounted so as to be rotatable about its longitudinal central axis and which likewise have, distributed about their periphery, cylinder bores in which the pistons can move. In order to adjust the volume flow, the cylinder drum is pivotable about a pivot axis which runs transversely with respect to its rotational axis, as a result of which the longitudinal central axis of the cylinder drum forms an adjustable angle with the axis of the driveshaft. The pistons are articulatedly supported here on the drive flange of the shaft at an adjustable angle. The cylinder block is for this purpose held by a correspondingly designed, rotatable end housing which is for example formed in the manner of a yoke or by the end housing, is mounted on a fixed housing part and can be pivoted by means of a servo system.
Swashplate units have a rotating cylinder drum with pistons which are distributed about the periphery and which move in the cylinders and which are supported in a sliding fashion on a swashplate. The pivot angle of the swashplate can be adjusted by means of a servo system. With increasing pivot angle, the piston stroke in the cylinder bores, and therefore the volume flow, increase.
The above-mentioned prior application DE 10 2006 025 347 describes a simplified, compact design of a hydrostatic transmission which is composed in each case of a pump and a hydraulic motor which, in the case of the swashplate design, are adjusted by means of a dual swashplate which has in each case one separate contact face with a different angle of inclination with respect to the shaft axis for the displacement pistons of the two units. In the case of the bent axis design, a pivotable dual yoke is provided, to which the pump and hydraulic motor are likewise positively coupled, which are thereby controlled together. Here, the hydraulic motor strokes in each case from a maximum to the minimum pivot angle while the pump is stroked from the minimum to the maximum pivot angle.
FIGS. 1 to 3 show said older system in a perspective illustration and in a front view and a side view. The two rotational axes 2, 3 of the shafts of the axial piston power units intersect the pivot axis 1 of the dual yoke 20 at right angles. All three axles lie in the same plane. Mounted in the dual yoke 20 are the cylinder blocks of the two units which, when the dual yoke 20 pivots, are likewise pivoted about the pivot axis 1. The side view in FIG. 3 also illustrates the servo pistons 4, 5 of the servo system which brings about the change in displacement. From this, it can be clearly seen that, with said arrangement of the pivot and rotational axes 1, 2, 3 on account of the given lever arms, it is possible to realize relatively low control torques, such that the servo pistons 4, 5 can be kept small.
However, with said geometry of the pivot and rotational axes 1, 2, 3, it is necessary to accept a significant power loss which is caused substantially by decompression losses. This is explained in more detail on the basis of FIGS. 4 to 6. FIG. 4 shows a cross section through an axial piston unit at the maximum pivot angle of 45°. Here, the design can be such that the piston 6, in its end position, extends up to a short distance in front of the end of the piston bore in the cylinder block 7. The remaining space between the power unit piston 6 and the bore end is then at a minimum, such that the decompression volume also remains at a minimum. The remaining oil which is at high pressure is connected to the low pressure during the further rotation of the cylinder block 7 and expands in the low pressure. Since said space can be kept small at the maximum pivot angle, the decompression losses are low here.
At a pivot angle of 22.5°, half of the maximum pivot angle, the piston 6 no longer reaches the base of the bore at dead centre. As shown in FIG. 5, the dead volume 9 is now enlarged considerably and is already 25% of the maximum displacement of the piston unit. At a pivot angle of 0°, as illustrated in FIG. 6, the dead volume 10 is then 50% of the maximum displacement. Here, especially, there are very high decompression losses which increase quadratically with pressure and, at a pressure of over 250 bar, are of very high significance. For example, the power loss of a 250 cm3 pump at a pivot angle of 0° and at a rotational speed of 4000 rev/mm and a pressure of 400 bar is approximately 10 KW.